An R—Fe—B magnet containing an R2Fe14B intermetallic compound as a main phase, wherein R is at least one of rare earth elements including Y, is usually plated because of poor oxidation resistance. Though plating metals are generally nickel, copper, etc., the R—Fe—B magnet is eroded by a nickel plating solution in direct contact, because the nickel plating solution is acidic. Accordingly, it is general to form a nickel plating layer on the surface of the R—Fe—B magnet after forming a copper plating layer thereon as a primer layer.
From the aspect of improving adhesion to a magnet substrate and preventing pinholes, a copper cyanide has conventionally been used for the copper plating (Japanese Patent Laid-Open No. 60-54406). However, because copper cyanide is extremely toxic, the highest attention should be paid to the safety of production, the control of plating solutions, and the treatment of waste water. In view of the recent trend of avoiding materials harmful to the environment, a copper plating method using no copper cyanide is desired.
Known as electrolytic copper plating solutions for R—Fe—B magnets are plating solutions of copper pyrophosphate, copper sulfate and copper borofluorate in addition to a plating solution of copper cyanide. It has been found, however, that when these electrolytic copper plating solutions are used for R—Fe—B magnets, metal elements in the R—Fe—B magnets are dissolved or subjected to a substitution reaction, resulting in electrolytic copper plating layers have poor adhesion to the R—Fe—B magnet and magnets without high thermal demagnetization resistance.
The electroless plating of R—Fe—B magnets is also carried out. Proposed as an electroless plating method in Japanese Patent Laid-Open No. 8-3763 is a method for forming an electroless copper plating layer as a first layer, an electrolytic copper plating layer as a second layer, and an electrolytic nickel-phosphorus plating layer as a third layer on an R—Fe—B magnet. However, because the first layer is an electroless copper plating layer in this method, it is not only poor in adhesion to the R—Fe—B magnet, but also it is easily self-decomposed because it is more unstable than the electrolytic plating solution.
Incidentally, as a method for forming an electrolytic copper plating not on an R—Fe—B magnet but in through-holes of a printed wiring board, Japanese Patent Laid-Open No. 5-9776 proposes a method for forming an electrolytic copper plating at a current density of 0.2-2.0 A/dm2, using a plating solution at pH of 8-10, which contains 30-60 g/liter (hereinafter referred to as “g/L”) of a chelating agent, 5-30 g/L of copper sulfate or a copper chelate compound, 50-500 ppm of a surfactant, and 0.5-5 cm3/liter of a pH-buffering agent. However, in the electrolytic copper plating method using an electrolytic copper plating solution at pH of 8-10, it has been found that an electrolytic copper plating layer formed on the R—Fe—B magnet suffers from pinholes, and that the electrolytic copper plating layer has poor adhesion to the R—Fe—B magnet.
If there were slightest pinholes in the copper plating layer, the R—Fe—B magnet would gradually be oxidized, losing its desired magnetic properties. Also, poor adhesion to the R—Fe—B magnet causes the peeling of the copper plating layer from the R—Fe—B magnet, resulting in the oxidation of the R—Fe—B magnet.
Further, when the copper plating layer has a Vickers hardness lower than the predetermined level, small dents of about 50-500 μm are disadvantageously formed on the surface of the copper plating layer by the collision of the copper-plated R—Fe—B magnets with each other, etc., resulting in poor appearance and corrosion resistance.